1. Field of the Invention
The present invention relates generally to an electronic circuit and, more particularly, to an output driver for efficiently transmitting data to a reception system.
2. Description of the Related Art
A variety of signals, including data signals, are transmitted from a transmission system to a reception system via a transmission line. In general, the transmission system includes an output driver so as to efficiently transmit data. It is very important to design the output impedance of the output driver so that it has a value corresponding to the transmission impedance of the transmission line and the reception impedance of the reception system. Furthermore, in order to emphasize a transmission signal in the initial stage of driving, a pre-emphasis technique for generating a separate auxiliary signal has been widely used. Using such a pre-emphasis technique, the efficiency of the transmission of a transmission signal can be improved. It is very important to match the amount of impedance of an auxiliary output terminal for generating the auxiliary signal, and the driving width and driving time of the auxiliary signal to the amount of impedance of a main output terminal for generating the transmission signal, and the driving width and driving time point of transmitted data, respectively. If appropriate matching is not achieved, a transmission signal may be distorted.
FIG. 1 is a block diagram of a prior art output driver. In FIG. I, a main driving circuit 10 drives a main signal XMDR to a transmission line 1 in response to a data signal DAT. The output terminal of the main driving circuit 10 has main impedance MIM. Furthermore, the PMOS transistor 21 of an auxiliary driving circuit 20 pulls up an auxiliary signal XSDR in response to a pull-up control signal/UPCON output from a pull-up generation means 25.
Furthermore, the NMOS transistor 23 of the auxiliary driving circuit 20 pulls down the auxiliary signal XSDR in response to a pull-down control signal DNCON output from a pull-down generation means 27. The auxiliary driving circuit 20 is operated to drive the auxiliary signal XSDR in the same direction as the main signal XMDR, in the initial stage of transmission of the data signal DAT.
As a result, the main signal XMDR based on the data signal DAT is emphasized by the auxiliary signal XSDR in the initial stage of driving, and is created as an output signal DOUT.
In the output driver of FIG. 1, auxiliary impedance SIM, that is, the impedance at the output terminal of the auxiliary driving circuit 20, is fixed. Therefore, output impedance OIM, which is formed by the combination of the main impedance MIM and the auxiliary impedance SIM, is fixed.
Accordingly, the prior art output driver has a problem in that it is very difficult to match the output impedance OIM to the transmission impedance TIM of the transmission line 1 and the reception impedance RIM of a reception system 2.
Furthermore, in the output driver of FIG. 1, the initial driving time point of the auxiliary signal XSDR is fixed. Therefore, the prior art output driver has a problem in that it is difficult to control the pre-emphasis time point of an output signal.
Furthermore, in the output driver of FIG. 1, the initial driving width of the auxiliary signal XSDR is also fixed. Therefore, the prior art output driver also has a problem in that it is difficult to control the pre-emphasis width of an output signal.